What spark plugs did you use when you replaced them? Only use the original NGK spark plugs the heat range is higher than what other brands cross them to. The also have the last few threads removed so that they do not protrude through the head which will build up carbon on the threads if you use the wrong ones and most likely pull the threads out of the head when removing. Another issue is what grade of LP fuel you are using, most LP fuel suppliers, use the lower grades and the vaporizers do not like the sludge build up. If you are not familiar with rebuilding one of these vaporizers I would leave it to a pro or you can buy remanufactured versions from your local Cat dealer.